1. Field of the Invention
The present invention generally relates to an inter-ear or binaural correlation coefficient correcting apparatus. It particularly relates to a correction circuit in which a coefficient of correlation between the right and left ears of a listener especially in a car is corrected so as to eliminate unnatural sound localization.
2. Background of the Invention
A time interval between when the right and left ears of a person receive a sound wave has a close relationship to the frequencies of the sound waves which reach the ears. That is, a phase difference of sound waves which reach the right and left ears is specifically affected by a low frequency component of a wavelength substantially the same as the interval between the right and left ears and has a peculiar directional pattern. A person can recognize a spatial sound impression owing to a binaural level difference, a binaural phase difference, a directional pattern, etc., of sound waves which reach both ears.
Such a binaural correlation may be repesented by the following expression of a binaural correlation coefficient .rho..sub.LR : ##EQU1## where the symbols P.sub.L (t) and P.sub.R (t) represent sound pressure applied to the right and left ears respectively, and [P.sub.L (t)] and [P.sub.R (t)] represent time averages of these sound pressures.
When the expression (1) is applied to a general listening room, the value of the binaural correlation coefficient .rho..sub.LR is about 1, that is, sound waves transmitted to the right and left ears are in phase within a lower medium frequency sound range, whereas in a high frequency sound range, on the contrary, there is a tendency that the value of the binaural correlation .rho..sub.LR approaches zero because the sound wavelength in a high sound range is shorter than the interval between the right and left ears so that there is no correlation in the phase relationship.
When the above-mentioned expression (1) is applied to the inside of a car compartment, the binaural correlation coefficient .rho..sub.LR at a listener's sitting position in the car compartment takes such a measured value as shown by a dotted line in FIG. 3, owing to the reflection of sounds in the car compartment and asymmetry of the sound source and of the sound space with respect to the listener's sitting position. As apparent from the measured value shown in FIG. 3, there arises a phenomenon in a lower medium frequency sound range that the value of the binaural correlation coefficient .rho..sub.LR becomes negative, that is, the phase between the right and left ears is inverted. This phase inversion causes sound twining unclear feeling of sound localization, so that the listener feels that the sounds are unpleasant.
In order to correct such a .rho..sub.LR characteristic as described above, a technique shown in FIG. 7 has been proposed. In FIG. 7, there are provided two signal paths. A signal applied to an input terminal 7 of one of the signal paths is simultaneously fed to each of a low-pass filter 1 and high-pass filters 2 and 4, which separate a frequency range into two frequency bands. Further, the high-pass filter 2 together with a low-pass filter 3 constitute a band-pass filter to allow necessary frequency components to pass therethrough so as to be applied to a phase inverter circuit 5 which inverts the phase of the signal components in this frequency band. The signals passed through the respective filters are added to one another by an adder 6 and the output of the adder 6 is applied to an output terminal 9.
The other signal path allows the signal to pass therethrough as it is from an input terminal 8 to an output terminal 10.
By using such a correction circuit, it is possible to provide a phase difference to signals applied to a right and a left speaker. However, a filter circuit for separating the band to be corrected must be used so that not only is the circuit made complicated but undulations are caused in a frequency characteristic of the signals when the signals are added in the adder 6.